The present invention relates to a surface coating, and, more particularly relates to a urethane dispersion that is useful for application to inorganic substrates to provide a chemically and environmentally resistant coating that exhibits a high gloss appearance.
Inorganic, metallic and mineral substrates are particularly susceptible to the deleterious effects of environmental and chemical elements. For example, natural stones such as marble stain easily and are vulnerable to citrus fruit, carbonated beverages, acids, alcohol, and other chemical solvents which etch the surface. In addition, natural stones can be scratched by hard or sharp objects such as broken glass. Therefore, a protective surface coating is commonly applied to marble vanities, bars, desk tops and tables to protect the marble surface and enhance its polished appearance.
Polymeric resin dispersions are commonly utilized as coating compositions, wherein the composition is applied to the substrate surface whereupon the resin cures to provide a protective coating. One problem associated with various resin dispersions is the presence of highly volatile organic solvents which create environmental and safety hazards. To combat the hazards associated with solvent based coating compositions aqueous polymeric dispersions have been developed which generally undergo a humidity or moisture cure. A moisture cure essentially involves the complete evaporation of the aqueous solvent and cross-linking of the resin to form a hard polymeric coating.
This invention is directed to a moisture curable aqueous urethane dispersion useful as a surface coating composition for inorganic substrates. Other aqueous polymer resin dispersions, based on as vinyl derivatives or acrylics, have proven less resistant to chemical and environmental elements, such as ketones, sea salt, acid rain and ultra violet radiation than some urethanes. In the past, urethane dispersions have been suggested for use on inorganic materials. However, particularly in the case of mineral substrates such as granite, fired or adobe brick, marble, limestone, slate or other mineral substrates, a composition that is protective, and which exhibits good adhesion over a period of time without discoloration is lacking in the art.
In order to achieve a glossy and protective finish, the coating composition must exhibit good adhesion to the substrate. This is somewhat problematic when working with mineral substrates due to surface porosity and the latent or subsurface hydroscopic nature of these materials. In the past, a thick primer coat or sealant material has been applied to the natural stone surface prior to application of a top coat. Although the sealant covers the pores and provides a smooth surface for top coat adherence, discoloration and decay of the underlying stone and peeling of the sealant material results due to developed vapor pressure from the latent and subsurface trapped water. Although a coating composition is preferably resistant to water and liquid materials, some degree of breathability or permeability to water vapor is required in the case of mineral substrates since water retention at the surface is common. An ideal coating for inorganic and mineral substrates is therefore both chemically and physically protective while offering some vapor permeation.
U.S. Pat. No. 4,810,533 to Smith discloses utilizing moisture curable urethane resins as a coating materials for application to inorganic substrates. However, not all moisture cured urethane dispersions provide a protective adherent coating. Aromatic urethane dispersions tend to progressively form double bonds over time and in the presence of ultra violet light, which causes the coating to become yellow and brittle. Although non-aromatic, thinner urethane coatings are available, they continue to exhibit poor adhesion to inorganic mineral substrates. To realize a smooth glossy finish, the coating composition must flow easily and uniformly over the substrate surface upon application and subsequently cure and harden without stress ripples, checks, blemishes or bubbles. It is known that surfactants can be used to reduce surface tension and promote uniform flow. Although the wettability of the substrate is enhanced through the use of surfactants, when water dispersible urethanes are used in association with surfactant blends, the loss of water from the resin elements during the moisture cure is often interfered with and substrate adhesion is reduced by the presence of these surfactants.
Surfactants by definition exhibit their desirable properties by preferentially concentrating at surfaces or interfaces, which can prevent adhesion of the composition to the substrate. For example, typical surfactants include alkylated sulfonates or alkylaryl sulfonates. The sulfonate groups are highly soluble in water and when these groups are exposed to water or humidity at the substrate interface, they may solubilize in the water and interfere with permanent resin adhesion. Therefore it is difficult to obtain a smooth and attractive finish that exhibits good adhesion.
To further complicate this problem, sulfonate groups commonly provided in a variety of surfactants tend to progressively form double bonds, especially in oxidizing and ultraviolet light environments, which causes brittleness and yellowing at the interfacial substrate/resin bond.
Therefore, it is an object of the present invention to provide a surface coating, for inorganic metallic and mineral substrates, having a very thin, smooth and fine finished texture without ripples, checks or stippling and which demonstrates extremely good adhesion at the substrate interface and at any secondary interfacial layers.
It is another object of the present invention to provide a surface coating that flows easily and uniformly over potentially large areas of the substrate during application such that a smooth, high gloss appearance is attained.
It is a further object of the present invention to provide a surface coating that is both hard and flexible and which is resistant to physical, chemical and environmental degradation.
It is also an object of the present invention to provide a surface coating which behaves as a fluid barrier while exhibiting water vapor permeability.
It is yet another object of the present invention to provide a surface coating which exhibits excellent adhesion on porous substrates that are hydroscopic in nature.
It is a further object of the present invention to provide a surface coating that is not susceptible to degradation by ultraviolet light, salt, citric acid, alcohol, solvents and other chemicals.
It is also an object of the present invention to provide a surface coating for application to inorganic metallic surfaces and mineral substrates, including natural stone such as marble, fired and adobe brick, granite, limestone and slates.
These and other objects of the present invention are achieved by a surface coating composition comprising a moisture-curable aqueous urethane resin, a cross-linking agent and an anionic surfactant, wherein the surfactant has a phosphate group substituent. The water-borne resin is provided as an aqueous dispersion of discrete linear aliphatic urethane chains, wherein the resin chains have two or more pendent acid group substituents. The cross-linking agent functions to effect a thorough resin cure by co-reaction with the pendent acid group substituents during the curing process.
In this invention, interfacial binding, adhesion and stability are enhanced by addition of an anionic surfactant having a phosphate substituent which in a sequential process reduces surface tension, specifically binds to positively charged substrate entities and, upon increasing dryness, provides a substrate interface that is relatively hydrophobic. This process can be demonstrated on positively charged substrates of metal, certain plastics, and mineral compositions.
When this surfactant is combined with the urethane resin of the invention, resin cross-linking during dehydration, film hardening, interfacial adhesion and chemical and environmental resistance are enhanced beyond the expectations of conventional coating technologies. Unexpectedly, the combination of surfactant and aqueous resin complex is highly effective even on surfaces which have latent or bound water and on polished surfaces which have proven extremely difficult to coat with the prior art compositions.
This invention additionally focuses on a method for applying the surface coating composition of the present invention to inorganic materials and in particular encompasses a method for applying the composition to natural stone materials such as marble, fired and adobe brick, granite, limestone and other inorganic substrates. Prior to application of the composition, the substrate surface can be treated to uniquely enhance the ability of the stone surface to interact and bond strongly with the anionic surfactant of the present invention. In another embodiment, the substrate is first coated with a composition comprising the aqueous urethane dispersion and surfactant heretofore mentioned, while excluding the cross-linking agent. A second coat is then applied, wherein the coating composition contains the cross-linking agent. This method results in a uniform coating having superior interlayer resin binding without a discernible interface or weakness between the two layers.
A very thin coating prepared in accordance with this invention has a high gloss and smooth appearance. The surface is hard, exhibiting no evidence of fisheyes, rippling, orange peel or yellowing over time and with exposure to the elements. The finish is not susceptible to degradation by lemon juice, alcohol, UV light, salt spray and several other chemicals. The inventors have formulated a composition that exhibits superior flow upon application to inorganic substrates and adheres strongly to the substrate.